The present invention relates to methods for efficiently forming optical devices in glass utilizing deep UV light ( less than 300nm). Specifically, the invention relates to direct-write methods of forming light guiding structures in glass compositions through light-induced refractive index changes. The invention also relates to the optical devices made by the direct-write methods. The invention also relates to bulk glass substrate bodies in which densified waveguides can be directly and efficiently written.
Optical devices such as optical waveguides and Bragg diffraction gratings are widely known in the telecommunications field. In an optical waveguide, a higher refractive index core surrounded by a lower refractive index cladding guides light and can transmit a large amount of optical information over long distances with little signal attenuation. The optical waveguide fiber is the prototype device of this type. The fiber is produced by a method that, by virtue of its fabrication from different material core glasses and different material cladding glass with high and low refractive indexes, gives the proper waveguiding structure. A Bragg grating is another type of an optical device that can be used to filter and isolate a narrow band of wavelengths from a broader signal. The most common materials used commercially in telecommunications applications of light guiding devices are doped silica-based compositions such as germania doped silica core and pure dry hydroxyl-free silica clad.
It is known that laser sources can be used to effect both index changes and to produce physical damage in glass. With regard to the former, the use of a pulsed UV radiation laser source for writing Bragg gratings in germania doped silica core fibers is known. Recently, a xe2x80x9cdirect-writexe2x80x9d laser method of forming optical waveguides within a glass volume that is transparent to the wavelength of a femtosecond laser has been disclosed. In this method, a 120 fsxe2x89xa6pulsed 810-nm laser is focused within a polished piece of silica as the glass is translated perpendicular to the incident beam through the focus. Increases in refractive index on the order of 10xe2x88x922 were reported for a specific condition in which the focus was scanned ten times over the exposed area.
One potential problem with a direct write process of forming waveguides in bulk glass using short-pulse focused lasers is over-exposure. Irradiation with too much energy can lead to physical damage in the glass. Physical damage and break down of the glass results in undesired attenuation of optical signals transmitted through the glass.
Another problem in direct write methods of making optical structures relates to the trade-off between the dimensional stability of the writing device, e.g., the laser, and the energy necessary to induce the desired refractive index change in the substrate material.
To make the laser direct-write method industrially practical, changes in the refractive index of a material must be achieved in a reasonable amount of writing time. There continues to be a need for a practical direct write method of creating silica-based optical devices having a sufficiently increase refractive index at an acceptably high write rate. Such a method could be used to write continuous light-guiding waveguide patterns connecting any two points within a continuous block of a suitable material, or make other optical devices, such as Bragg gratings.
Silica-germania is often used as a material whose index can be altered with light. In the photosensitive response of germania doped silica, H2-loading is typically employed as a method to increase the response of the glass. In that situation, the mechanism of index increase is by color center formation mechanism, through the Kramer-Kronig relationship. The use of H2-loading introduces logistical issues, including time required to load with H2. for bulk materials, in particular, the time to impregnate H2 at a temperature low enough that the H2 does not react with the material, becomes prohibitively long. For example, a 3-mm thick piece of a silicate glass takes 36 days to load at 150xc2x0 C. Once having gotten the H2 into the material, the storage of pieces containing H2 becomes an issue, although for bulk material this is less of an issue.
The increase in refractive index through densification offers advantages in material handling in that H2 loading is not necessary. Hydrogen loading may be practical for small dimensional glass structures such as optical fibers and thin planar layers where hydrogen can be readily diffused in, it is impractical for three dimensional addressing and writing in the interior of larger glass bodies because of the problems and difficulties in diffusing the hydrogen deep inside the glass body interior and absorption properties of the glasses. Another potential advantage of the invention is that thermal stability of a feature written using H2-loaded silica-germania typically undergo a thermal anneal at low temperature, during which the grating is xe2x80x9cfixed.xe2x80x9d This thermal anneal decreases the efficiency of the grating by about 30% but helps to insure that further changes in efficiency are minimized. The inventive utilization of the densification mechanism yields a feature that needs no thermal treatment for xe2x80x9cfixingxe2x80x9d and is generally more thermally robust in that features remain in the piece even at several hundred degrees, while providing writing deep into the interior of glass bodies that have depths from their surface that can be greater than 2 cm.
It is known that sustained high-energy radiation and laser exposure of high purity fused silica such as with excimer laser microlithography systems where laser beam exposure can produce a measurable densification effect in fused silica optical elements.
It is an object of the invention to provide improved direct-write methods of forming light guiding structures within a silica-based material substrate. In particular, it is desired to internally and efficiently write three-dimensional light guiding structures in glass, such as waveguides and gratings. A focussed deep UV ( less than 300 nm) laser beam is translated through the interior of a large dimension glass body to form densified glass waveguiding core structures through the glass interior with the densified glass waveguiding core structures able to traverse the glass body in three dimensions in multiple directions, through multiple planes and to multiple exterior surfaces of the glass body.
The inventive method includes internal direct write densification formation of waveguide cores within large glass bodies that have depths from the glass surface to the glass body interior of at least 1 cm, preferably at least 2 cm, preferably at least 3 cm, and most preferably at least 4 cm. The invention includes making optical waveguide devices in three dimensional glass bodies with direct written densified waveguide cores with interior non-surface corepath parts that are at least 1 cm, preferably at least 2 cm, preferably at least 3 cm, and most preferably at least 4 cm away from the exterior surfaces of the glass body.
It is a further object of the invention to provide method of writing optical structures in silica-based materials.
In accordance with one aspect of the invention, it has been discovered that soft silica-based materials exhibit increased sensitivity to laser writing of optical structures in the bulk.
In accordance with another aspect of the invention, a method is provided to directly write light guiding structures in glass using lasers with substantially no physical damage of the glass.
In accordance with another aspect of the invention, a method is provided to write three dimensional optical structures in silica-based bulk glass. Specifically, the invention provides for translating the refractive index-increasing focus of a laser through a silica-based substrate in the x, y, and z-dimensions.
In accordance with still another aspect of the invention, a variety of optical devices are disclosed which incorporate optical structures made by the methods described herein. The invention includes selectively densifying traced internal volume regions within a larger bulk volume of a soft (annealing point less than 1350xc2x0 K) silica glass with a  less than 300 nm deep UV laser beam focus to form densified optical waveguide core tunnels.
These and other aspects of the invention will become apparent to those skilled in the art in light of this disclosure.